nfsd4: make sure set CB_PATH_DOWN sequence flag set

Make sure this is set whenever there is no callback channel.

If a client does not set up a callback channel at all, then it will get
this flag set from the very start.  That's OK, it can just ignore the
flag if it doesn't care.  If a client does care, I think it's better to
inform it of the problem as early as possible.

Reported-by: Rick Macklem <rmacklem@uoguelph.ca>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>